Today, telecommunication signals are commonly transmitted using cellular systems. Cellular systems comprise groups of cellular base stations, each of which is used to transmit signals to and receive signals from a mobile device, such as a cellular telephone, laptop computer or other such mobile device. In addition to transmitting a variety of voice and/or data signals between the mobile device and the base station, cellular systems are often used to locate such mobile devices, both for emergencies and non-emergencies. For example, in the case of a call to the emergency number 911, it is frequently helpful to determine the location of the caller so that assistance can be dispatched to the caller immediately and without requiring that the caller know his/her location. In non-emergency cases, it is frequently desirable to determine the location of a mobile device to provide services such as roadside assistance, turn-by-turn driving directions, concierge services, location-specific billing rates and location-specific advertising, among others. The location of the mobile device is typically determined through trilateration, a process of establishing distance to three or more known reference points through the measurement of elapsed time of a non-directional signal between the mobile device and each known reference point and then plotting the unique intersection of those three or more solutions.
The CDMA protocol operates using a variety of channels. A Forward CDMA channel carries user and signaling traffic, a pilot signal, and overhead information, from a base station to a mobile device. The pilot and overhead signals establish the system timing and station identity. The pilot channel also provides a signal strength reference that is used in the mobile-assisted handoff (MAHO) process. All base stations have the same pilot waveform and are distinguished from one another only by the phase of the pilot signal.
Various techniques have been used to determine the location of a mobile device. For example, Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS), the GLONASS owned by the Russian Federation Government, and Galileo Radio Navigation Satellite System, are satellite systems that provide users equipped with a GNSS receiver the ability to determine their location anywhere in the world. A GNSS receiver normally determines its location by measuring the relative times of arrival of signals transmitted simultaneously from multiple GNSS satellites. In GNSS-deprived areas, such as indoors, under a heavy canopy and near tall buildings, GNSS equipped mobile devices fail to acquire signals from any or from a sufficient number of GNSS satellites to provide an accurate location.
In GNSS-deprived areas, another well-known position location technique such as Advanced Forward Link Trilateration (AFLT) may be used. The AFLT technique is based on measuring time-of-arrival differences between terrestrial base station pilot signals. In the case of a CDMA wireless network, these measurements are called pilot phase measurements.
It is possible that at a particular location a sufficient number of GNSS satellites and multiple CDMA pilot signals to calculate a fix are not available or the calculated location does not provide sufficient accuracy. Position determination capability is compromised at these locations.